(2)
The error signal required to position the rebalance/feedback system is developed in a dc bridge. Refer to figure 4. The air temperature probe forms one leg of the bridge and the rebalance potentiometer does the balancing. DC bridge balance is monitored by a solid-state chopper and when a changing resistance in the temperature probe unbalances the bridge, the chopper converts the imbalance to an error signal to drive the feedback loop. Mechanical motion in the loop then balances the dc bridge by repositioning the rebalance potentiometer to a null condition.
(3)
Positive and negative temperature indication is controlled by the mechanical position of the rebalance/feedback system. As shown in figure 4, system operation is reversed by switching the polarity of the dc bridge power supply when temperature readings pass through zero.
(4)
A yellow OFF flag is held out of view by an energized solenoid controlled by a failure monitor as shown in Fig. 4. The failure monitor allows the flag to drop into view upon sensing an excessive null voltage in the reposition/feedback system or the loss of 26-volt ac instrument power.
EFFECTIVITY
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C. The pilots' total air temperature indicator is powered from 115-volt ac flight instrument bus No. 2 through flight instrument transformer No. 6 and circuit breaker CADC 2, PILOTS TAT IND on panel P7. The flight engineer's total air temperature indicator is powered from the essential flight instrument bus, transformer No. 3 and through CADC 1, F/E TAT IND on panel P7.
D. Other than the circuit breaker on panel P7 controlling instrument power, the indicator has no external controls. Its operational modes are either on or failed as governed by the failure monitor.
5. Instantaneous Vertical Velocity Indicators (Fig. 1, Sheet 2)
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A. Instantaneous vertical velocity indicators provide an anticipated indication of airplane speed during ascent or descent. Internal accelerometers and static pressure from the pitot-static system supply inputs from which vertical speed is determined. Vertical speed is displayed by a pointer rotating in front of a graduated scale. Level flight is indicated by the pointer indicating zero, to the left. (Refer to sheet 2, figure 1.) The instrument indicates ascent by upward rotation of the pointer and descent by downward rotation of the pointer. The rate of ascent or descent is read on the indicator scale from 0 to 6000 feet-per-minute. The indicator is a pneumatically controlled mechanical system with no failure monitor or automatic failure mode. It requires no electrical connections other than scale illumination and is always on.
B. Instantaneous indication of airplane vertical speed is accomplished by combining the effects of an accelerometer and an air leak across a diffuser (a pneumatic resistance) to move a pressure sensitive diaphragm. The pointer position is controlled by diaphragm movement through linkage and a gear train.
(1) Static pressure enters; 1) the accelerometer assembly directly and 2) the case interior through a diffuser which delays response to static pressure change. The accelerometer assembly consists of two cylinders (dashpots) containing pistons held in balance by springs and their own weight. A change in vertical speed displaces the pistons inside the dashpots creating an immediate pressure change inside the diaphragm. Pressure outside the diaphragm remains the same, for the moment, and the resulting pressure differential produces an instantaneous indication by the instrument pointer. Accelerometer response decays after a few seconds but a pressure differential has built up across the diffuser because altitude (and hence static pressure) has changed. If vertical speed remains constant, the pressure differential is maintained and the instrument continues to display vertical velocity in feet-per-minute.
C. The zero adjustment screw on the lower left corner of the instrument face is used to reposition the pointer to zero. It mechanically moves the pointer from 400 feet-per-minute up to 400 feet-per-minute down. No other controls are associated with instrument.
6. Indicated Airspeed/Mach Indicators (Fig. 1, Sheet 1)
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A. Each airspeed indicator displays four airspeed functions through four independent devices within the instrument.
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航1(77)
